This invention is related to an FM stereo demodulator, and move particularly to an abnormal sound-preventing circuit in an FM stereo demodulator.
In an FM stereo demodulator circuit employing a PLL (phase locked loop), a voltage-controlled oscillator circuit (hereinbelow abbreviated to "VCO") constituting the PLL circuit receives an output signal of a phase detector in the PLL as a control signal and delivers an oscillation signal whose frequency and phase are controlled with regard to a pilot signal included in an FM detection signal. A reproduced subcarrier signal of 38 KHz for FM stereo demodulation is obtained on the basis of the oscillation signal.
The self oscillation frequency of the VCO, however, is ordinarily affected by a supply voltage and fluctuates according to a variation in the supply voltage even when the level of the control signal is constant.
Essentially, such fluctuation of the oscillation frequency is corrected by the output of the phase detector in the PLL. The correction, however, becomes impossible when the supply voltage drops drastically, causing the oscillation frequency to approach or exceed the boundary value between the lock range and the capture range of the PLL. When the frequency of the VCO has fluctuated in this manner, the frequency and phase of the reproduced subcarrier signal fluctuate accordingly, and a beat signal develops between the reproduced subcarrier signal and a subcarrier signal component in a composite signal of broadcast reception. As a result, an abnormal sound (noise) due to the beat signal comes to be included in each reproduced stereo output signal, i.e., right and left signals which are obtained by, for example, switching the composite signal with the reproduced subcarrier signal.
Such an abnormal sound results in a case where the supply voltage has conspicuously dropped on account of, for example, a high voice output, or in a case where a battery whose electromotive force has dropped is used in a receiver such as a portable radio set which operates at a low voltage. Even in a receiver which operates at a comparatively high voltage, in case where the supply voltage varies slowly when turning a power switch "on" or "off", the noise results in the course of the rise or fall of the supply voltage.